While a large nose is the mark of a witty, courteous, affable, generous and liberal man (at least according to Cyrano de Bergerac), an excellent sense of smell is the mark of one of nature's most exquisite senses. Olfactory sensing is exquisite because of its sensitivity, ability to detect many odorants, and ability to distinguish between odorants.
Smell has been exploited in many ways, notably to the detection of contraband and planted explosive devices. Dogs can be trained to detect unusual substances, such as explosives or narcotics. Hunting dogs search for game and search dogs search for missing people. Cadaver dogs are used to detect human remains. More recently, dogs have been successfully trained to smell out disease (Willis et al., 2004). Although dogs and their scent detecting capabilities have been extremely useful, their usefulness is limited by the need for expensive (up to tens of thousands of dollars) and time-consuming (often 2 years or more) training and limited lifetimes. Like humans, dogs can only work during limited times during the day before needing to recuperate. Human experts in various fields can also accurately detect and identify scents. For example, physicians can detect disease and conditions from the smells emitted by their patients, leading to quick and accurate diagnoses. The person on the street can easily detect a drunk by the characteristic ketones that occur as a result of alcohol metabolism, although this example is less than subtle.
The detection of odorants has also been pursued through the development of electronic noses that are used for environmental monitoring, medical testing, and food and drink production. In the most sophisticated systems, a unique chemical fingerprint can be generated by an array of sensors and then identified by pattern-recognition techniques, such as the smell of a rose (Lundstrom, 2000). Attempts to measure odors with electronic instruments were made in the 1950s, but the modern field of artificial olfaction, according to Lundstrom (Lundstrom, 2000), began in 1982 with the work of Persaud and Dodd (Persaud and Dodd, 1982). They used a small array of gas-sensitive metal-oxide devices to classify odors. While there has been a steady increase in the number of systems using chemical sensor arrays, their success depends not only on the development of new sensor technologies, but also on the availability of powerful pattern-recognition software (Lundstrom, 2000). This last aspect is particularly important for sensor arrays that produce a composite response—for detecting targets that emit a characteristic array of odorants. However, these systems suffer from many limitations that are superseded by the olfactory cells in animals.
Other examples of electronic noses, or “sniffers” include an elaborate system incorporating a testing chamber lined with arrays of gas sensors; to detect the odorants where the air pressure is lowered to create a draft into the chamber. The air that then rushes in carries the scent of the object to be identified to the sensors that detect emitted gases (Gelperin, 1997). The only gases that are detected, however, are those for which sensors are available. In another example of an electronic nose, the device consists of a chemically sensitive resistor electrically coupled to conductive elements (such as electrical leads). Arrays of these sensors in conjunction with an electrical measuring device are used to create an electronic nose for detecting analytes in fluids (Lewis and Severin, 1998). In some cases, the application of the sensor is highly specialized, such as those specifically adapted to assess the condition of cows by sensing odors emitting from their teats (Mottram and Wilkin, 1997), or to specifically detect microorganisms (Payne and Persaud, 1998).
Electronic noses have been helpful, but suffer from complex machinery and are often limited to specific applications. They require “training” to produce accurate pattern recognition. Like other calibrated instruments, electronic noses suffer from drift, causing changes in the sensor output which are not related to the presence of a stimulus and which can result in “false positive” errors. In addition, electronic sniffers suffer from fouling of the detection components, limited dynamic range, cross-sensitivity issues, and speed. Some of the complex machinery in electronic noses is required, for example, to provide reference odors, to compensate for sensor drift, or to increase the temperature using heaters in order to remove molecules from the sensor surface to refresh the surface. Circuitry or reference sensors sometimes need to be included to compensate for changes in temperature, humidity, and other environmental conditions.